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    <h1>Chapter 8: Mapped Diagnostic Context</h1>

    <a href="mdc_ja.html">&#x548C;&#x8A33; (Japanese translation)</a>

    <div class="quote">     
      <p><em>Lock the doors.</em></p>
      <p>&mdash;LEROY CAIN, Flight Director, Columbia Mission Control</p>
    </div>
    
    <script src="../templates/creative.js" type="text/javascript"></script>
    
		<p>One of the design goals of logback is to audit and debug
		complex distributed applications.  Most real-world distributed
		systems need to deal with multiple clients simultaneously.  In a
		typical multithreaded implementation of such a system, different
		threads will handle different clients. A possible but slightly
		discouraged approach to differentiate the logging output of one
		client from another consists of instantiating a new and separate
		logger for each client.  This technique promotes the proliferation
		of loggers and may increase their management overhead.
		</p>
		
    <script src="../templates/setup.js" type="text/javascript"></script>

    
    <p>A lighter technique consists of uniquely stamping each log
   	request servicing a given client. Neil Harrison described this
   	method in the book <em>Patterns for Logging Diagnostic
   	Messages</em> in Pattern Languages of Program Design 3, edited by
   	R. Martin, D. Riehle, and F. Buschmann (Addison-Wesley,
   	1997). Logback leverages a variant of this technique included in
   	the SLF4J API: Mapped Diagnostic Contexts (MDC).
		</p>
		
		<p>To uniquely stamp each request, the user puts contextual
		information into the <code>MDC</code>, the abbreviation of Mapped
		Diagnostic Context.  The salient parts of the MDC class are shown
		below. Please refer to the <a
		href="http://www.slf4j.org/api/org/slf4j/MDC.html">MDC
		javadocs</a> for a complete list of methods.
		</p>

<pre class="prettyprint source">package org.slf4j;

public class MDC {
  //Put a context value as identified by <em>key</em>
  //into the current thread's context map.
  <b>public static void put(String key, String val);</b>

  //Get the context identified by the <code>key</code> parameter.
  <b>public static String get(String key);</b>

  //Remove the context identified by the <code>key</code> parameter.
  <b>public static void remove(String key);</b>

  //Clear all entries in the MDC.
  <b>public static void clear();</b>
}</pre>

		<p>The <code>MDC</code> class contains only static methods.  It
		lets the developer place information in a <em>diagnostic
		context</em> that can be subsequently retrieved by certain logback
		components. The <code>MDC</code> manages contextual information on
		a <em>per thread basis</em>.  Typically, while starting to service
		a new client request, the developer will insert pertinent
		contextual information, such as the client id, client's IP
		address, request parameters etc. into the
		<code>MDC</code>. Logback components, if appropriately configured,
		will automatically include this information in each log entry.
		</p>

    <p>Please note that MDC as implemented by logback-classic assumes
    that values are placed into the MDC with moderate frequency.  Also
    note that a child thread does not automatically inherit a copy of
    the mapped diagnostic context of its parent.</p>

		<p>
			The next application named 
			<code><a href="../xref/chapters/mdc/SimpleMDC.html">SimpleMDC</a></code> 
			demonstrates this basic principle.
		</p>
<em>Example 7.1: Basic MDC usage (<a href="../xref/chapters/mdc/SimpleMDC.html">
logback-examples/src/main/java/chapters/mdc/SimpleMDC.java)</a></em>
<pre class="prettyprint source">package chapters.mdc;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.MDC;

import ch.qos.logback.classic.PatternLayout;
import ch.qos.logback.core.ConsoleAppender;

public class SimpleMDC {
  static public void main(String[] args) throws Exception {

    // You can put values in the MDC at any time. Before anything else
    // we put the first name
    MDC.put("first", "Dorothy");

    [ SNIP ]
    
    Logger logger = LoggerFactory.getLogger(SimpleMDC.class);
    // We now put the last name
    MDC.put("last", "Parker");

    // The most beautiful two words in the English language according
    // to Dorothy Parker:
    logger.info("Check enclosed.");
    logger.debug("The most beautiful two words in English.");

    MDC.put("first", "Richard");
    MDC.put("last", "Nixon");
    logger.info("I am not a crook.");
    logger.info("Attributed to the former US president. 17 Nov 1973.");
  }

  [ SNIP ]

}</pre>

		<p>The main method starts by associating the value
		<em>Dorothy</em> with the key <em>first</em> in the
		<code>MDC</code>. You can place as many value/key associations in
		the <code>MDC</code> as you wish.  Multiple insertions with the
		same key will overwrite older values.  The code then proceeds to
		configure logback.</p>

    <p>For the sake of conciseness, we have omitted the code that
    configures logback with the configuration file <a
    href="http://github.com/qos-ch/logback/blob/master/logback-examples/src/main/java/chapters/mdc/simpleMDC.xml">simpleMDC.xml</a>. Here
    is the relevant section from that file.
    </p>

 <pre class="prettyprint source">&lt;appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender"> 
  &lt;layout>
    &lt;Pattern><b>%X{first} %X{last}</b> - %m%n&lt;/Pattern>
  &lt;/layout> 
&lt;/appender></pre>
    


    <p>Note the usage of the <em>%X</em> specifier within the
    <code>PatternLayout</code> conversion pattern. The <em>%X</em>
    conversion specifier is employed twice, once for the key named
    <em>first</em> and once for the key named <em>last</em>. After
    obtaining a logger corresponding to <code>SimpleMDC.class</code>,
    the code associates the value <em>Parker</em> with the key named
    <em>last</em>.  It then invokes the logger twice with different
    messages.  The code finishes by setting the <code>MDC</code> to
    different values and issuing several logging requests. Running
    SimpleMDC yields:
		</p>

<div class="source"><pre>Dorothy Parker - Check enclosed.
Dorothy Parker - The most beautiful two words in English.
Richard Nixon - I am not a crook.
Richard Nixon - Attributed to the former US president. 17 Nov 1973.</pre></div>


		<p>The <code>SimpleMDC</code> application illustrates how logback
		layouts, if configured appropriately, can automatically output
		<code>MDC</code> information.  Moreover, the information placed
		into the <code>MDC</code> can be used by multiple logger
		invocations.
		</p>
		
		<h3 class="doAnchor">Advanced Use</h3>
		
		<p>Mapped Diagnostic Contexts shine brightest within client server
		architectures.  Typically, multiple clients will be served by
		multiple threads on the server.  Although the methods in the
		<code>MDC</code> class are static, the diagnostic context is
		managed on a per thread basis, allowing each server thread to bear
		a distinct <code>MDC</code> stamp. <code>MDC</code> operations
		such as <code>put()</code> and <code>get()</code> affect only the
		<code>MDC</code> of the <em>current</em> thread, and the children
		of the current thread. The <code>MDC</code> in other threads
		remain unaffected. Given that <code>MDC</code> information is
		managed on a per thread basis, each thread will have its own copy
		of the <code>MDC</code>.  Thus, there is no need for the developer
		to worry about thread-safety or synchronization when programming
		with the <code>MDC</code> because it handles these issues safely
		and transparently.
		</p>

		<p>The next example is somewhat more advanced.  It shows how the
		<code>MDC</code> can be used in a client-server setting.  The
		server-side implements the <code>NumberCruncher</code> interface
		shown in Example 7.2 below. <code>The NumberCruncher</code>
		interface contains a single method named
		<code>factor()</code>. Using RMI technology, the client invokes
		the <code>factor()</code> method of the server application to
		retrieve the distinct factors of an integer.
		</p>

<em>Example 7.2: The service interface (<a href="../xref/chapters/mdc/NumberCruncher.html">
logback-examples/src/main/java/chapters/mdc/NumberCruncher.java)</a></em>
<pre class="prettyprint source">package chapters.mdc;

import java.rmi.Remote;
import java.rmi.RemoteException;

/**
 * NumberCruncher factors positive integers.
 */
public interface NumberCruncher extends Remote {
  /**
   * Factor a positive integer <code>number</code> and return its
   * <em>distinct</em> factor's as an integer array.
   * */
  int[] factor(int number) throws RemoteException;
}</pre>

		<p>
			The <code>NumberCruncherServer</code> application, listed in Example 7.3 below, 
			implements the <code>NumberCruncher</code> interface. Its main method exports 
			an RMI Registry on the local host that accepts requests on a well-known port.  
		</p>

<em>Example 7.3: The server side (<a href="../xref/chapters/mdc/NumberCruncherServer.html">
logback-examples/src/main/java/chapters/mdc/NumberCruncherServer.java)</a></em>
<pre class="prettyprint source">package chapters.mdc;

import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.rmi.server.UnicastRemoteObject;
import java.util.Vector;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.MDC;

import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.classic.joran.JoranConfigurator;
import ch.qos.logback.core.joran.spi.JoranException;


/**
 * A simple NumberCruncher implementation that logs its progress when
 * factoring numbers. The purpose of the whole exercise is to show the
 * use of mapped diagnostic contexts in order to distinguish the log
 * output from different client requests.
 * */
public class NumberCruncherServer extends UnicastRemoteObject
  implements NumberCruncher {

  private static final long serialVersionUID = 1L;

  static Logger logger = LoggerFactory.getLogger(NumberCruncherServer.class);

  public NumberCruncherServer() throws RemoteException {
  }

  public int[] factor(int number) throws RemoteException {
    // The client's host is an important source of information.
    try {
      <b>MDC.put("client", NumberCruncherServer.getClientHost());</b>
    } catch (java.rmi.server.ServerNotActiveException e) {
      logger.warn("Caught unexpected ServerNotActiveException.", e);
    }

    // The information contained within the request is another source
    // of distinctive information. It might reveal the users name,
    // date of request, request ID etc. In servlet type environments,
    // useful information is contained in the HttpRequest or in the  
    // HttpSession.
    <b>MDC.put("number", String.valueOf(number));</b>

    logger.info("Beginning to factor.");

    if (number &lt;= 0) {
      throw new IllegalArgumentException(number +
        " is not a positive integer.");
    } else if (number == 1) {
      return new int[] { 1 };
    }

    Vector&lt;Integer> factors = new Vector&lt;Integer>();
    int n = number;

    for (int i = 2; (i &lt;= n) &amp;&amp; ((i * i) &lt;= number); i++) {
      // It is bad practice to place log requests within tight loops.
      // It is done here to show interleaved log output from
      // different requests. 
      logger.debug("Trying " + i + " as a factor.");

      if ((n % i) == 0) {
        logger.info("Found factor " + i);
        factors.addElement(new Integer(i));

        do {
          n /= i;
        } while ((n % i) == 0);
      }

      // Placing artificial delays in tight loops will also lead to
      // sub-optimal results. :-)
      delay(100);
    }

    if (n != 1) {
      logger.info("Found factor " + n);
      factors.addElement(new Integer(n));
    }

    int len = factors.size();

    int[] result = new int[len];

    for (int i = 0; i &lt; len; i++) {
      result[i] = ((Integer) factors.elementAt(i)).intValue();
    }

    <b>// clean up
    MDC.remove("client");
    MDC.remove("number");</b>

    return result;
  }

  static void usage(String msg) {
    System.err.println(msg);
    System.err.println("Usage: java chapters.mdc.NumberCruncherServer configFile\n" +
      "   where configFile is a logback configuration file.");
    System.exit(1);
  }

  public static void delay(int millis) {
    try {
      Thread.sleep(millis);
    } catch (InterruptedException e) {
    }
  }

  public static void main(String[] args) {
    if (args.length != 1) {
      usage("Wrong number of arguments.");
    }

    String configFile = args[0];

    if (configFile.endsWith(".xml")) {
      try {
        LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory();
        JoranConfigurator configurator = new JoranConfigurator();
        configurator.setContext(lc);
        lc.reset();
        configurator.doConfigure(args[0]);
      } catch (JoranException je) {
        je.printStackTrace();
      }
    }

    NumberCruncherServer ncs;

    try {
      ncs = new NumberCruncherServer();
      logger.info("Creating registry.");

      Registry registry = LocateRegistry.createRegistry(Registry.REGISTRY_PORT);
      registry.rebind("Factor", ncs);
      logger.info("NumberCruncherServer bound and ready.");
    } catch (Exception e) {
      logger.error("Could not bind NumberCruncherServer.", e);

      return;
    }
  }
}</pre>

		<p>The implementation of the <code>factor(int number)</code>
		method is of particular relevance. It starts by putting the
		client's hostname into the <code>MDC</code> under the key
		<em>client</em>. The number to factor, as requested by the client,
		is put into the <code>MDC</code> under the key
		<em>number</em>. After computing the distinct factors of the
		integer parameter, the result is returned to the client. Before
		returning the result however, the values for the <em>client</em>
		and <em>number</em> are cleared by calling the
		<code>MDC.remove()</code> method. Normally, a <code>put()</code>
		operation should be balanced by the corresponding
		<code>remove()</code> operation. Otherwise, the <code>MDC</code>
		will contain stale values for certain keys. We would recommend
		that whenever possible, <code>remove()</code> operations be
		performed within finally blocks, ensuring their invocation
		regardless of the execution path of the code.
		</p>	
		
		<p>
			After these theoretical explanations, we are ready to run the number 
			cruncher example. Start the server with the following command:
		</p>
		
<div class="source"><pre>java chapters.mdc.NumberCruncherServer src/main/java/chapters/mdc/mdc1.xml</pre></div>
		
		<p>
			The <em>mdc1.xml</em> configuration file is listed below:
		</p>
<em>Example 7.4: Configuration file (logback-examples/src/main/java/chapters/mdc/mdc1.xml)</em>
<pre class="prettyprint source">&lt;configuration>
  &lt;appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
    &lt;layout>
      &lt;Pattern>%-4r [%thread] %-5level <b>C:%X{client} N:%X{number}</b> - %msg%n&lt;/Pattern>
    &lt;/layout>	    
  &lt;/appender>
  
  &lt;root level="debug">
    &lt;appender-ref ref="CONSOLE"/>
  &lt;/root>  
&lt;/configuration></pre>

		<p>
			Note the use of the <em>%X</em> conversion specifier within the 
			<span class="option">Pattern</span> option.
		</p>
	
		<p>
			The following command starts an instance of <code>NumberCruncherClient</code> 
			application:  	
		</p>
		
<div class="source"><pre>java chapters.mdc.NumberCruncherClient <em>hostname</em></pre></div>

		<p>
			where <em>hostname</em> is the host where the 
			<code>NumberCruncherServer</code> is running
		</p>
		
		<p>
			Executing multiple instances of the client and requesting the server to factor 
			the numbers 129 from the first client and shortly thereafter 
			the number 71 from the second client, the server outputs the following:
		</p>
		
<div class="source"><pre>
<b>70984 [RMI TCP Connection(4)-192.168.1.6] INFO  C:orion N:129 - Beginning to factor.</b>
70984 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 2 as a factor.
71093 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 3 as a factor.
71093 [RMI TCP Connection(4)-192.168.1.6] INFO  C:orion N:129 - Found factor 3
71187 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 4 as a factor.
71297 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 5 as a factor.
71390 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 6 as a factor.
<b>71453 [RMI TCP Connection(5)-192.168.1.6] INFO  C:orion N:71 - Beginning to factor.</b>
71453 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 2 as a factor.
71484 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 7 as a factor.
71547 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 3 as a factor.
71593 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 8 as a factor.
71656 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 4 as a factor.
71687 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 9 as a factor.
71750 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 5 as a factor.
71797 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 10 as a factor.
71859 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 6 as a factor.
71890 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 11 as a factor.
71953 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 7 as a factor.
72000 [RMI TCP Connection(4)-192.168.1.6] INFO  C:orion N:129 - Found factor 43
72062 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 8 as a factor.
72156 [RMI TCP Connection(5)-192.168.1.6] INFO  C:orion N:71 - Found factor 71</pre></div>

		<p>
			The clients were run from a machine called <em>orion</em> as can be seen in 
			the above output. Even if the server processes the requests of clients 
			near-simultaneously in separate threads, the logging output pertaining 
			to each client request can be distinguished by studying the output of the 
			<code>MDC</code>. Note for example the stamp associated with <em>number</em>, 
			i.e. the number to factor. 
		</p>
		
		<p>
			The attentive reader might have observed that the thread name could 
			also have been used to distinguish each request. The thread name can cause 
			confusion if the server side technology recycles threads. In that case, 
			it may be hard to determine the boundaries of each request, that is, 
			when a given thread finishes servicing a request and when it begins servicing the next.
			Because the <code>MDC</code> is under the control of the application developer, 
			<code>MDC</code> stamps do not suffer from this problem. 
		</p>
		
		
		
		<h3 class="doAnchor" name="autoMDC">Automating access to the <code>MDC</code></h3>
		
		<p>As we've seen, the <code>MDC</code> is very useful when dealing
			with multiple clients. In the case of a web application that
			manages user authentication, one simple solution could be to set
			the user's name in the <code>MDC</code> and remove it once the
			user logs out. Unfortunately, it is not always possible to
			achieve reliable results using this technique. Since
			<code>MDC</code> manages data on a <em>per thread</em> basis, a
			server that recycles threads might lead to false information
			contained in the <code>MDC</code>.
		</p>
		
		<p>To allow the information contained in the <code>MDC</code> to
			be correct at all times when a request is processed, a possible
			approach would be to store the username at the beginning of the
			process, and remove it at the end of said process. A servlet <a
			href="http://java.sun.com/javaee/5/docs/api/javax/servlet/Filter.html">
			<code>Filter</code></a> comes in handy in this case.
		</p>
		
		<p>Within the servlet filter's <code>doFilter</code> method, we
		can retrieve the relevant user data through the request (or a
		cookie therein), store it the <code>MDC</code>.  Subsequent
		processing by other filters and servlets will automatically
		benefit from the MDC data that was stored previously. Finally,
		when our servlet filter regains control, we have an opportunity to
		clean MDC data.
		</p>
		
		<p>Here is an implementation of such a filter:</p>

<em>Example 7.5: User servlet filter (<a href="../xref/chapters/mdc/UserServletFilter.html">
logback-examples/src/main/java/chapters/mdc/UserServletFilter.java)</a></em>
<pre class="prettyprint source">package chapters.mdc;

import java.io.IOException;
import java.security.Principal;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;

import org.slf4j.MDC;

public class UserServletFilter implements Filter {

  private final String USER_KEY = "username";
  
  public void destroy() {
  }

  public void doFilter(ServletRequest request, ServletResponse response,
    FilterChain chain) throws IOException, ServletException {

    boolean successfulRegistration = false;

    HttpServletRequest req = (HttpServletRequest) request;    
    Principal principal = req.getUserPrincipal();
    // Please note that we could have also used a cookie to 
    // retrieve the user name

    if (principal != null) {
      String username = principal.getName();
      successfulRegistration = registerUsername(username);
    } 

    try {
      chain.doFilter(request, response);
    } finally {
      if (successfulRegistration) {
        MDC.remove(USER_KEY);
      }
    }
  }

  public void init(FilterConfig arg0) throws ServletException {
  }
  

  /**
   * Register the user in the MDC under USER_KEY.
   * 
   * @param username
   * @return true id the user can be successfully registered
   */
  private boolean registerUsername(String username) {
    if (username != null &amp;&amp; username.trim().length() > 0) {
      MDC.put(USER_KEY, username);
      return true;
    }
    return false;
  }
}</pre>

	<p>When the filter's <code>doFilter()</code> method is called, it
	first looks for a <code>java.security.Principal</code> object in the
	request. This object contains the name of the currently
	authenticated user. If a user information is found, it is registered
	in the <code>MDC</code>.
	</p>
		
	<p>Once the filter chain has completed, the filter removes the user
		information from the <code>MDC</code>.
	</p>

  <p>The approach we just outlined sets MDC data only for the duration
  of the request and only for the thread processing it. Other threads
  are unaffected. Furthermore, any given thread will contain correct
  MDC data at any point in time.</p>
		


  <h3 class="doAnchor" name="managedThreads">MDC And Managed
  Threads</h3>

  <p>A copy of the mapped diagnostic context can not always be
  inherited by worker threads from the initiating thread. This is the
  case when <code>java.util.concurrent.Executors</code> is used for
  thread management. For instance, <code>newCachedThreadPool</code>
  method creates a <code>ThreadPoolExecutor</code> and like other
  thread pooling code, it has intricate thread creation logic.
  </p>

  <p>In such cases, it is recommended that
  <code>MDC.getCopyOfContextMap()</code> is invoked on the original
  (master) thread before submitting a task to the executor.  When the
  task runs, as its first action, it should invoke
  <code>MDC.setContextMapValues()</code> to associate the stored copy
  of the original MDC values with the new <code>Executor</code>
  managed thread.
  </p>

  <h3 class="doAnchor" name="mis">MDCInsertingServletFilter</h3>

  <p>Within web applications, it often proves helpful to know the
  hostname, request uri and user-agent associated with a given HTTP
  request. <a
  href="../xref/ch/qos/logback/classic/helpers/MDCInsertingServletFilter.html"><code>MDCInsertingServletFilter</code></a>
  inserts such data into the MDC under the following keys.
  </p>

  <table class="bodyTable">
    <tr>
      <th>MDC key</th>
      <th>MDC value</th>
    </tr>

    <tr  class="alt">
      <td><code>req.remoteHost</code></td>
      <td>as returned by the <a
      href="http://java.sun.com/j2ee/sdk_1.3/techdocs/api/javax/servlet/ServletRequest.html#getRemoteHost%28%29">getRemoteHost()</a>
      method
      </td>
    </tr>

    <tr >
      <td><code>req.xForwardedFor</code></td>
      <td>value of the <a
      href="http://en.wikipedia.org/wiki/X-Forwarded-For">"X-Forwarded-For"</a>
      header
      </td>
    </tr>

    <tr class="alt">
      <td><code>req.method</code></td>
      <td>
        as returned by <a
        href="http://java.sun.com/j2ee/sdk_1.3/techdocs/api/javax/servlet/http/HttpServletRequest.html#getMethod%28%29">getMethod()</a>
        method
      </td>
    </tr>

    <tr>
      <td><code>req.requestURI</code></td>
      <td>
        as returned by <a
        href="http://java.sun.com/j2ee/sdk_1.3/techdocs/api/javax/servlet/http/HttpServletRequest.html#getRequestURI%28%29">getRequestURI()</a>
        method
      </td>
    </tr>

    <tr class="alt">
      <td><code>req.requestURL</code></td>
      <td>
        as returned by <a
        href="http://java.sun.com/j2ee/sdk_1.3/techdocs/api/javax/servlet/http/HttpServletRequest.html#getRequestURL%28%29">getRequestURL()</a>
        method
      </td>
    </tr>

    <tr>
      <td><code>req.queryString</code></td>
      <td>
        as returned by <a
        href="http://java.sun.com/j2ee/sdk_1.3/techdocs/api/javax/servlet/http/HttpServletRequest.html#getQueryString%28%29">getQueryString()</a>        method
      </td>
    </tr>

    <tr class="alt">
      <td><code>req.userAgent</code></td>
      <td>value of the "User-Agent" header
      </td>
    </tr>

  </table>

  <p>To install <code>MDCInsertingServletFilter</code> add the
  following lines to your web-application's <em>web.xml</em> file</p>

  <pre class="prettyprint source">&lt;filter>
  &lt;filter-name>MDCInsertingServletFilter&lt;/filter-name>
  &lt;filter-class>
    ch.qos.logback.classic.helpers.MDCInsertingServletFilter
  &lt;/filter-class>
&lt;/filter>
&lt;filter-mapping>
  &lt;filter-name>MDCInsertingServletFilter&lt;/filter-name>
  &lt;url-pattern>/*&lt;/url-pattern>
&lt;/filter-mapping> </pre>

  <p><b>If your web-app has multiple filters, make sure that
  <code>MDCInsertingServletFilter</code> is declared before other
  filters.</b> For example, assuming the main processing in your
  web-app is done in filter 'F', the MDC values set by
  <code>MDCInsertingServletFilter</code> will not be seen by the code
  invoked by 'F' if <code>MDCInsertingServletFilter</code> comes after
  'F'.
  </p>

  <p>Once the filter is installed, values corresponding to each MDC
  key will be output by the %X <a
  href="layouts.html#conversionWord">conversion word</a> according to
  the key passes as first option. For example, to print the remote
  host followed by the request URI on one line, the date followed by
  the message on the next, you would set <code>PatternLayout</code>'s
  pattern to:
  
  </p>

  <p class="source">%X{req.remoteHost} %X{req.requestURI}%n%d - %m%n</p>

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